Conjugates of cytotoxic T lymphocytes attached to their target cells were studied by double immunofluorescence on fixed smears to detect simultaneously the localization of actin and myosin within the cells. Actin was found to be polarized in the area of attachment of the lymphocytes (but not of the target cells), whereas myosin remained evenly distributed in the cytoplasm. When living conjugates were brought to 37 degrees C to induce cytotoxicity, this pattern remained unchanged, but observation by interference reflexion microcinematography revealed a high motility of the lymphocytes in the contact area. This localized motility in the area of attachment associated with a peculiar actin polarization, which has no equivalent in any type of cell contact presently known, could represent a necessary step in the sequence of events leading to target cell killing by cytotoxic T lymphocytes.